Serotonergic modulation of glutamate neurotransmission as a strategy for treating depression and cognitive dysfunction.

Abstract

Monoamine-based treatments for depression have evolved greatly over the past several years, but shortcomings such as suboptimal efficacy, treatment lag, and residual cognitive dysfunction are still significant. Preclinical and clinical studies using compounds directly targeting glutamatergic neurotransmission present new opportunities for antidepressant treatment, with ketamine having a surprisingly rapid and sustained antidepressant effect that is presumably mediated through glutamate-dependent mechanisms. While direct modulation of glutamate transmission for antidepressant and cognition-enhancing actions may be hampered by nonspecific effects, indirect modulation through the serotonin (5-HT) system may be a viable alternative approach. Based on localization and function, 5-HT can modulate glutamate neurotransmission at least through the 5-HT1A, 5-HT1B, 5-HT3, and 5-HT7 receptors, which presents a rational pharmacological opportunity for modulating glutamatergic transmission without the direct use of glutamatergic compounds. Combining one or more of these glutamate-modulating 5-HT targets with 5-HT transporter inhibition may offer new therapeutic opportunities. The multimodal compounds vortioxetine and vilazodone are examples of this approach with diverse mechanisms, and their different clinical effects will provide valuable insights into serotonergic modulation of glutamate transmission for the potential treatment of depression and associated cognitive dysfunction.

A schematic diagram of the hypothesized modulatory role of 5-HT receptors on glutamatergic neurotransmission. A glutamatergic pyramidal neuron and several GABA interneurons expressing the 5-HT3, 5-HT1A, 5-HT7, and 5-HT1B receptors on either dendrites or axon terminals are shown. The multimodal compounds vortioxetine and vilazodone and their possible sites of action are also shown. Note that 5-HT1A, 5-HT1B, and 5-HT7 receptors may be localized on different neuronal populations. Symbols used: VLA, vilazodone; VOR, vortioxetine.